its existence critical yet seemingly transient in the overall life cycle of the sea urchin. The skeleton of the pluteus does, however, give rise to the spines of the juvenile sea urchin. These spines usually measure 1-3 centimeters in length and 1-2 millimeters thick, and in some species, may be poisonous. Molecular regulation The molecular mechanisms of skeletogenesis involve several PMC-specific gene products. These include Msp30, a sulfate cell-surface glycoprotein which has been implicated in calcium uptake and deposition, and SM50, SM30, and PM27 which are three proteins of the spicule matrix. SM50 and PM27 are thought to be structurally similar, nonglycosylated, basic proteins whereas SM30 is an acidic glycoprotein. The specific roles of these matrix proteins has yet to be fully elucidated, but it is thought that they may function in the nucleation or orientation of crystal growth. It has also been found that the msp130 gene exhibits a complex pattern of spatial regulation within the PMC syncytium during skeletogenesis. It is suggested that the ectoderm may play a role in controlling skeletal morphogenesis by regulating the expression of PMC-specific gene products involved in spicule biogenesis. Evolution The extent to which the molecular mechanisms underlying skeletogenesis in larval sea urchins has been characterized has led to comparative evolutionary developmental studies in distantly-related sea urchins, as well as other echinoderms, with the aim of understanding how this character has evolved. These studies, and others, have revealed that numerous differences have arisen during the evolution of the sea urchin clade in spatiotemporal gene expression of several transcription factors comprising the gene regulatory network driving skeletogenic specification. However, there are also striking similarities in the signaling systems that position these cells in the embryo. Despite differences in timing of mesodermal ingression into the blastocoel and spatiotemporal differences in transcription factor gene expression, ancestral state reconstruction of genes critical to the specification of sea urchin skeletogenic cells supports the homology of this cell type, suggesting it arose some time before the divergence of cidaroids and euechinoids over 268 million years ago. References Echinoidea Animal developmental biology